Towards Spatial Impression: A Unified Listener’s Envelopment Metric—The Periíchisi Index

The primary goal in designing classical music concert halls is to create acoustic and spatial conditions that foster deep listener immersion, a phenomenon often termed acoustic envelopment or “spaciousness”. International standards recognize this phenomenon with the parameters of Lateral Energy Fraction and Late Lateral Gain. The present research applies a cross-disciplinary methodology to investigate whether a unified index can be formulated to describe holistically the perception of a listener being immersed in and surrounded by the acoustic sound field within a concert hall, hereafter termed Periichisi. The research question is examined through three specific components: the exploration of the acoustic, psychoacoustic, and possible neurological mechanisms underlying this perceptual condition; the identification of architectural and musical factors that contribute to its formation; and the evaluation of selected acoustic parameters, including G, C80, and BR, in supporting the proposed index. The research methodology uses an ex post facto analysis of psychoacoustic data collected at the AuroLab chamber in the School of Architecture, NTU Athens. To promote objectivity and inclusivity, the dataset comprises 130 participants, intentionally including non-expert listeners without formal musical training. Key contributions include the introduction of the term Periíchisi into the Hellenic technical lexicon and the development of the Periíchisi Index for quantifying listener envelopment. Findings suggest future directions, including EEG-based studies of brain activity during Periíchisi and the strategic optimization of G, C80, and BR to enhance immersive acoustic experiences.